The purity of ethylene feedstock is critical to the quality of a wide range of petrochemical products, particularly polyethylene plastics. Yet, periodically there are problems when contaminant concentrations exceed stringent purity specifications, rendering valuable ethylene feedstock unacceptable for use in producing such materials.

One of the most problematic of those impurities is acetylene (C2H2), which can be difficult to measure accurately using conventional analyzers, and can also drift off spec due to slow analyzer response.

Because it is one of the more problematic contaminants that can spoil ethylene production, acetylene is removed from the ethylene during the purifying proves via the Acetylene Converter Unit. This series of reactors converts the acetylene in the hydrogen to form ethylene. When performing accurately, the converter reduces the amount of acetylene down to low PPM or even PPB levels as required.

“Acetylene hinders and damages the catalyst used in the chemical reaction to form polyethylene,” says Al Kania, a veteran of the petrochemical industry. “A big part of the problem is that acetylene, like moisture, is a component that is very difficult to measure accurately through chemical analysis. Also, the few tools that were previously available, such as the gas chromatograph, were unable to go very low in terms of sensitivity.”

Today, however, there is a technology for accurately measuring acetylene content in ethylene. That technology is a sophisticated application of the tunable diode laser (TDL), that was first developed by NASA to measure gas species even in extremely low concentrations.

TDL versus GC

There are several advantages of the TDL-based analyzer over the gas chromatograph (GC), which has been the primary tool for measuring acetylene and other impurities in ethylene until now.

To begin with, the TDL analyzer is extremely accurate, and remains so for the life of the analyzer. It is quite capable of measuring the concentrations of acetylene and other impurities at sub parts per million levels. The TDL analyzer, developed by NASA-spinoff SpectraSensors, Inc., Houston, TX, has become a state-of-the-art tool for accurately and continuously measuring very low levels of moisture, H2S and other impurities in hydrocarbon streams for several years.

The accuracy of the TDL analyzer is especially applicable to measuring acetylene impurities in ethylene today, not only because ethylene is such a valuable commodity, but also because excessive levels of such impurities will corrupt downstream uses of the ethylene.

“In the past, processors used to be able to tolerate a fair amount of acetylene in ethylene. But in polyethylene production, the presence of acetylene above stringent PPM and sometimes PPB levels is unacceptable and creates very expensive waste,” adds Kania.

If an acetylene converter unit operates too aggressively, he says, it could hydrogenate some of the ethylene back into ethane. So, the proper operation of the unit requires the fast and precise monitoring of acetylene in between the first of the two acetylene reactors (mid-bed) and as it exits the final reactor.

In addition to dependable and repeatable accuracy, the speed of measurements is vital to maintaining consistent purity in ethylene production. TDL analyzer offers the advantage of very high-speed readings.

When using the GC technology, the readings are relatively slow, commonly taking from 3 – 6 minutes to provide measurements. Conversely, the TDL analyzer provides almost continuous readings, with intervals taking only from 1-4 seconds.

“When you’re producing a lot of ethylene every day, acetylene and other impurities can swing on you very quickly,” Kania explains. “If you are unaware of those impurities for several minutes, which happens with the GC, you don’t have a chance to reroute the corrupted offspec ethylene. Also, it will take many hours of production, perhaps days, to purge it out of their downstream units. That’s an expensive proposition. On the other hand, the TDL analyzer measures the ethylene as it is manufactured to ensure it is of a very high purity.”

Beyond measurement

In addition to high-speed accuracy, the TDL laser offers ethylene producers other significant benefits over the traditional GC technology.

GCs are traditionally very complex mechanically, and require a lot of labour-based maintenance, Kania says. They also require a lot of consumables in order to maintain proper operation. However, the TDL analyzer requires very little maintenance and no consumables.

“In the past this was not so much of an issue because the ethylene was so valuable that processors simply bit the bullet and put in whatever resources were necessary,” explains Kania. “But the TDL analyzer requires only periodic maintenance, which adds to valuable uptime. And it does not use consumables, which may be secondary, but also adds to the high ROI.”

Another advantage of the TDL analyzer is the ability of ethylene producers to standardize on equipment. In addition to acetylene, there is an assortment of contaminants they need to monitor, and the TDL analyzer is an ideal solution for a large number of these contaminants.

The SpectraSensors analyzer, for instance, measures H2O, NH3, H2S and CO2 as well as acetylene. It is possible for users to standardize on SpectraSensors analyzers, operating them from the same control center.

“When they use multiple units for various contaminants they can roll up the measurements,” Kania says. “By doing this they not only upgrade their analyzer systems but also create more stable operations so that they don’t have to worry about these impurities or over purifying.”